A US man was fired from his overnight job at a supermarket after he intervened to help a woman allegedly being assaulted in the store's car park.

Kristopher Oswald, 30, was fired from Walmart this week, WXYZ-TV reports. "The last thing I expected was to not have a job," says Mr Oswald, who thought he was doing the right thing in going to the woman’s aid.

Mr Oswald, who worked stocking pet food on the graveyard shift at the Missouri store, was taking his meal break in his car around 2.30am when he heard a woman screaming and saw a man hanging onto the bonnet of her car.

When he approached to check on the woman’s safety, the man jumped on Mr Oswald, screaming "I’m going to kill you".

Just when Mr Oswald thought he had the man under control, two other men jumped him from behind, he says.

Police arrived on the scene but Mr Oswald’s employment was terminated, with the company stating: "after a violation of company policy on his lunch break, it was determined to end his temporary assignment".

Mr Oswald says Walmart has policies against workplace violence to prevent employees from assaulting co-workers or tackling a shoplifter, but it appears that nothing allows for staff to assist in situations of imminent danger and self-defence.

Mr Oswald only began working for Walmart about seven weeks ago and would not have been considered a permanent hire until a 180-day probation period ended.

Walmart say they have since reviewed the facts and understand Oswald's intentions were good.

When asked if he would take his job at Walmart back, Oswald sighed, hesitated and then said that he wasn't sure.